The present invention relates to a duct, in particular for high voltages, for connecting an electrical device isolated with gas, for example a transformer, a throttle coil, a measuring transformer, a capacitor or a switching device, with a connecting part located in atmospheric air, with at least one tubular field control electrode inside a gas-filled bushing insulator.
Such a duct is disclosed for example in the German document DE-PS 36 16 243. In this duct a cylindrical control capacitor composed of several control electrodes surrounds a cylindrical conductor. The capacitor is mounted with its lower electrode on a flange so that it forms a first chamber in its interior which is filled with sulfurhexafluoride (SF.sub.6) under high pressure as insulating gas. A second chamber is located outside of the capacitor and filled with the same gas under lower pressure. This known gas insulating duct has an explosion-protective construction in which the bushing insulator composed of porcelain is not directly subjected to the high pressure as long as the seal between the electrode and the capacitor and the individual electrodes relative to electrically insulated perforated discs maintain the overpressure of the gas.
The control electrodes are surrounded at both ends by ring-shaped connection electrodes and mounted by means of conical perforated discs on cast resin directly on one another so that a creep path extending cone is produced and no creep discharge due to the available potential difference occurs.
The known duct has a disadvantage that for avoiding creep discharge complicated holders of the control electrodes are needed. Since the distance between the high voltage electrode and the control electrode is bridged by insulating material, the space of the duct loaded with high field intensity no longer provides for legal requirements for a pure gas insulation and relatively great distances between the electrodes are required.
The German reference DE 28 00 208 describes a "Ceramic Sleeve Insulator with Pressure Gas Filling, in Particular for Electrical Devices and Apparatuses". This sleeve insulator is provided in its interior with a gas permeable sleeve which contains in its interior a pressure gas filler and during bursting of porcelain sleeve must prevent damages to the surrounding area. The sleeve insulator is mounted on a plate gas tightly and surrounds a control electrode which is also mounted there, through which a conductor rod extends in a housing under the plate. The conductor rod extends further at the upper end of the sleeve insulator through a further plate to the exterior.
The German reference DE 11 98 888 discloses a "High Voltage Duct" in which a current conductor is guided in an insulating hollow body which is filled with gaseous or liquid insulating material and the field distribution is influenced by an electrode which is conductively connected with a grounded frame and circularly surrounds the bushing conductor in the insulating body. The insulating hollow body composed of two parts, together with metal tubes, is connected with the grounded frame. First ring electrodes are located at the ends of the,metal tube, and second ring electrodes conductively connected with the bushing conductor are located opposite to the first ring electrodes. By the arrangement of this ring electrode pair and the shape of the insulating hollow body, the stresses in the axial direction must be favorably influenced.
The German document DE 37 40 86 describes an "Electrical Bushing Insulator" in which the electrodes are formed as metal coating on insulating bodies. Moreover, the German document DE 22 05 035 discloses mounting of a conductive coating on the surface of cylindrical insulating parts for forming electrodes in this manner.
The German document DE 18 00 667 finally describes a "Free Air-Duct with Pressure Gas Filling for High Voltage" which has a multi-part ceramic casing with control electrodes which is held by ring discs gas tightly clamped by neighboring parts of the casing (sleeve insulator). The control electrodes are arranged concentrically around a tubular conductor. The geometric shape and the position of the control electrodes is selected so that the potential distribution on the surface of the duct is at least approximately linear.
The disadvantage of this construction is however that the ring discs in this arrangement are not located in a field-poor region so that no pure gas insulation is provided in the highly stressed region. Since the ring discs are connected further with (metallic) screws for mounting on the casing parts with one another, the potential of the corresponding control electrode is drawn to the insulator surface, so that it is placed under a high stress.